Magnetic recording media are widely used in various formats such as audio tapes, video tapes, and floppy disks. A magnetic recording medium has a non-magnetic support on which is provided a magnetic layer having ferromagnetic powder dispersed in a binder.
A magnetic recording medium is required to satisfy high criteria of various characteristics including electromagnetic conversion, running durability, and running performance. In the recent data media applications where large capacity, high speed and high reliability are demanded, there is a fear that an MR head having a thin metal film undergoes deterioration of performance due to corrosion. It has been a subject to settle this problem.
As stated, not only excellent electromagnetic conversion characteristics but satisfactory running durability are required of a magnetic recording medium. To secure satisfactory running durability, an abrasive and a lubricant are usually incorporated into the magnetic layer.
To satisfy both the requirements of electromagnetic conversion and running durability, limitations of the size and number of abrasive particles to be used in the magnetic layer are proposed as in JP-A-2002-157726. An abrasive should be used in a substantial quantity in order to achieve substantial improvement on running durability, which results in, of necessity, a decrease of the proportion of the ferromagnetic powder. When an abrasive having a relatively large particle size is employed for obtaining excellent running durability, the abrasive particles are apt to project excessively from the magnetic layer surface. In other words, improvement on running durability by an abrasive can cause deterioration of the electromagnetic conversion characteristics.
In order to satisfy both electromagnetic conversion characteristics and running durability, it has been proposed to reduce the particle size of a magnetic substance and to optimize the amount of a lubricant as disclosed, e.g., in JP-A-2001-325716. A lubricant, when used for the purpose of improving running durability, must be added in an increased amount. It would follow that the binder is easily plasticized, which can result in reduced durability of the magnetic layer.
The binder, one of the main components of the magnetic layer, also plays an important roll in improving running durability and electromagnetic conversion characteristics, as will be appreciated from the following. It is known that a non-magnetic layer can be provided between a support and a magnetic layer so as to allow thinning of the magnetic layer. A magnetic recording medium with a still thinner magnetic layer and still finer ferromagnetic metal particles has been demanded. A finer ferromagnetic metal powder is less dispersible, and poor dispersibility results in deterioration of magnetic layer's surface properties and electromagnetic conversion characteristics and makes it difficult to assure durability.
That is, it is desirable for the binder resin used in such an improved magnetic recording medium to have high dispersing capabilities for ferromagnetic metal powder and non-magnetic powder and excellent durability based on hardness (i.e., high Tg and high Young's modulus) and toughness (or elongation).
Metal evaporated (ME) tape having a ferromagnetic metal thin film formed by vacuum evaporation has been put to practical use as video tape fit for digital video cassettes (DVCs) of consumer VCRs (SD format). Because metal particulate (MP) tape, whose magnetic layer is formed by applying a ferromagnetic metal powder dispersion, is superior to ME tape in durability and economy, it could be very useful in application to consumer DVCs. However, it has hardly been expected of MP tape to exhibit sufficiently stable electromagnetic conversion performance, especially overwrite capabilities, even when prepared using a binder resin having the above-described excellent properties.
It is conceivable means to decrease coercive force Hc and thickness of the magnetic layer in order to improve overwrite. That means, if adopted alone, results in reduction of saturated magnetic flux density Øm of the magnetic layer and reduction of read output. That is, in order to improve overwrite without inviting read output reduction, it is necessary to increase the packing density of magnetic particles in the magnetic layer by reducing the magnetic layer thickness or to raise the saturated magnetic flux density Øm of the magnetic layer by, for example, using a magnetic substance with high saturation magnetization σs. The problem confronting is that magnetic particles with increased σs are disturbed in magnetic orientation due to their magnetostatic interaction, which results in reduced output. A condition under which satisfactory read output and improved overwrite can be obtained stably has not been found as yet.
A magnetoresistive (MR) head provides a few times as much output as an inductive head. Having no inductive coil, an MR head achieves great reduction of noise created by equipment, such as impedance noise. Therefore, an MR head, being promising for improvement on high-density recording reproduction, has been steadily extending its application. Compared with a metal-in-gap (MIG) head that provides output signals in accordance with the saturated magnetic flux density Øm, an MR head has an advantage of high sensitivity. However, if an MR head is saturated, it fails to give satisfactory output only to have poor electromagnetic conversion performance.
Thus, the present inventors have extensively studied for obtaining excellent electromagnetic conversion in a read system using an MR head.